Laser scientists and engineers have long recognized that direct-diode lasers can offer compelling advantages over other laser technologies due to their efficiency, reliability, compactness and relatively low cost. These advantages have made diode lasers an enabling technology for high volume applications in telecommunications and data storage.
Applications for direct-diode lasers have been limited, however, due to their low brightness–a combination of lower power and poorer beam quality than alternatives. Their penetration into the most demanding industrial applications, including metal cutting and laser welding, has been poor. Instead, they’ve been relegated to niches like brazing, heat treating and paint stripping.
Using Beam Combining to Increase Brightness
There are three generally accepted approaches to increasing the brightness of direct-diode lasers:
Side-by-side beam combining (or spatial beam combination)
Coherent beam combining (CBC)
Wavelength beam combining (WBC)
Only WBC has the necessary qualities to be adapted for use in a high brightness industrial laser. Side-by-side beam combining increases output power in proportion to the number of emitters, but decreases beam quality by the same proportion. As a result, brightness can’t be increased. Coherent beam combining demands active phase locking of all emitters such that the optical path length difference between emitters is λ/10 or better. Little progress has been made in scaling CBC beyond several watts, despite substantial investment, so CBC is not considered a practical pathway to a high-brightness industrial laser at this time.
Wavelength beam combining is an incoherent, multi-wavelength process. When implemented in TeraDiode’s TeraDrive™ technology, output power scales in proportion to the number of emitters, while the beam quality of an individual emitter is preserved. Thus, brightness increases with the number of emitters.
WBC can be thought of as the spatial and directional superposition of many independent diode laser external cavities. The angle-to-wavelength conversion property of a diffraction grating is used to provide feedback to each emitter in an array, via a series of lenses, at different wavelengths. The laser resonator is formed between the HR coated back facet of the emitter and the output coupler. WBC allows for brightness scaling of an emitter array because all of the laser elements are spatially overlapped at the output coupler, maintaining the output beam quality of a single element while scaling the output power by the number of elements in the array.
With this fundamental breakthrough in WBC technology, TeraDiode has developed the first ultra-high brightness, direct-diode lasers that are bright enough to cut and weld metal with performance similar or better than the fiber laser. They combine unprecedented brightness with efficiency, reliability and low cost. At TeraDiode, we believe that direct-diode lasers using TeraDrive™ technology will, in time, replace fiber, disk and other lasers for the most demanding material processing applications.
The TeraDrive™ technology can be applied to any array of laser elements, over a wide range of power and wavelength combinations TeraDiode has demonstrated WBC using laser diode bars and stacks operating near one micron. Nevertheless, arrays of fiber, solid-state or gas lasers operating at wavelengths from the UV to mid IR range can also be used.
Combining the Advantages of WBC and Direct-Diode Lasers
A direct-diode laser using TeraDrive™ technology has all the advantages of WBC, including ultra-high spatial brightness, power scaling, high spectral brightness, wavelength stability and wavelength selectability. WBC also preserves the advantages of direct-diode lasers, including high efficiency and high reliability.
Ultra-high spatial brightness. TeraDiode’s fiber-coupled WBC direct-diode lasers provide output power of more than 2 kW with a BPP of 3 mm-mrad, equal to a brightness of 2,293 MW/cm2-sr. This is about fifty times brighter than commercially available 2 kW direct-diode lasers. Our kilowatt class direct-diode lasers for metal processing will deliver brightness of 6,800 MW/cm2-sr, which equals or exceeds that of comparable fiber lasers.
Power scaling. TeraDiode’s lasers range from several watts, powered by a few single emitters, to several kilowatts, powered by our TeraBlade™ product platform consisting of an array of diode bars. For directed energy weapons, the company has charted a development pathway that leads to a nearly diffraction limited, free-space 100 kW system.
High spectral brightness. TeraDrive™ technology can be optimized to deliver a narrow bandwidth, which is essential for efficient pumping of fiber and solid-state lasers, as well as DPALs.
Wavelength selectability. The TeraDrive™ technology works equally well with sources covering a wide range of wavelengths. For example, ultra-high brightness lasers can be constructed using production laser diodes. To date we have demonstrated operation at 780, 976 nm, 1550nm and 4µm wavelengths. Longer or shorter wavelengths can be obtained by using fiber, quantum cascade, or gas lasers as inputs.
Wavelength stability. For efficient pumping, the pump laser’s center wavelength must be relatively insensitive to changes in temperature and power. The wavelength locking characteristic of the TeraDrive™ means that dλ/dT and dλ/dP are both one to two orders of magnitude lower than typical diode pumps.
TeraDiode was featured in The Fabricator in the October 2012 edition.
TeraDiode was featured in Laser Focus World in June of 2012
TeraDiode was featured in Laser Focus World in March 2013